I am attempting to put up quite heavy floating shelves onto a plasterboard wall. I have used plasterboard raw plugs and have tightened the brackets with the rods as tightly as I can to the wall but when I place the shelves over the rods the shelves droop at the front. Is there anything I can do to prevent this?

Hi Jamesm:
You might try toggle screws; they go through the sheetrock and have a self-opening butterfly that gives more holding area on the back of the sheetrock.
Its too late to talk about it now, unless you want to patch some holes, but I would have screwed them to the studs behind the sheetrock. Hang in there!
Glenn